It In recent years, as virtual reality (VR) technologies mature, various VR helmets appear. The working principle of VR helmets is that images generated by small two-dimensional displays (such as LCD display screens) are amplified by optical systems. Specifically, light emitted from small displays passes convex lenses so that a seemingly distant effect is produced by refraction. At the same time, left and right eye screens display left and right images through left and right lenses respectively. After acquiring information with parallax by human eyes, stereoscopic images are generated in brains.
The optical systems of VR helmets inevitably introduce optical distortion and color aberration effects.
The cause of optical distortion is that the amplification rate of a lens changes along with changes of angles between light beams and a main axis of the lens and is not a constant value. Depending on the difference of the lenses of the optical system, barrel-shaped or pillow-shaped distortion may be produced; the closer to the edges of the lenses of the optical system, the greater the distortion is; and there is almost no distortion at the centers of the lenses of the optical system. Usually, pillow-shaped distortion is generated by optical systems of VR helmets.
The color aberration effect is caused by different wavelengths and refractive indexes of light of different colors. Among visible lights, red light has larger wavelengths and smaller refractive indexes, while blue light has smaller wavelengths and larger refractive indexes. Color aberration will produce two results: object points cannot properly focus into perfect image points, which leads to a blur of image; a rainbow effect may be generated at peripheral parts of images, and particularly at junctions of bright and dark (black and white) parts.
Elimination of optical distortion and color aberration effects may be realized by advanced distortion-eliminating achromatic lenses (or lens groups). But distortion and color aberration cannot be eliminated completely, and further correction and compensation using digital image processing technologies are needed. For example, for an optical system with pillow-shaped distortion, barrel-shaped distortion is performed to the original image first; and after the processed image passes the optical system with pillow-shaped distortion, distortion is offset. Then, human eyes can see normal images without distortion. Optionally, color aberration compensation may be performed through calculations using three sets of parameters for the three RGB colors. Currently, the most widely used correction algorithm is the higher order polynomial model algorithm (usually three orders or above are expanded). Calculations are carried out by substituting image correction parameters in the polynomials, and image correction is performed based on the calculation results.
Existing VR helmets can be divided into non-adjustable ones and adjustable ones. An adjustable VR helmet means that a physical structure of the helmet can be changed. For example, a distance between a left eye lens center and a right eye lens center, and a distance between a lens and a screen can be adjusted. So, an adjustable VR helmet is suitable for most users with different interpupillary distances or with myopic eyes or presbyopic eyes, for example. However, the inventor(s) of the present invention find(s) that for an adjustable VR helmet, as the image correction parameters do not change, the original image correction parameters are used after the structure of the helmet is adjusted, so that the image correction effect is degraded. Particularly, when the physical structure of the helmet is changed substantially, imaging will be affected significantly, and human eyes will directly feel image deformation and color aberration effects.